Continuously growing traffic density not only involves a relative increase in the risk of accident for single vehicles, but it also leads to a constantly rising number of insufficiently experienced drivers. The average driver usually misjudges vehicle behavior in emergency situations. The false estimation of road surface conditions alone or together with the misjudgment of the handling properties of the vehicle often causes serious accidents which could otherwise be avoided by an accurate estimation of the vehicle behavior and the road surface condition.
A special category of these accidents consists of those due to overbraking and resulting in skidding which can not be controlled by average drivers.
Anti-skid control methods therefore have been developed in order to enable drivers to optimally carry out braking operations even on wet and slippery road surfaces in the case of errors in braking. A survey of the known systems can be found in "Automotive Engineering", Vol. 81 (1973), No. 8, pages 27 to 37.
All these known methods use deceleration of one or more vehicle wheels as an indicator or incipient skidding behavior. In principle, control actions are initiated after an upper deceleration value has been exceeded at the wheel(s) to be controlled, e.g. by releasing the applied brake pressure until the skidding condition is avoided. Most of these known methods require expensive electronic means and complicated hardware to perform their control functions. These prior art anti-skid control systems have proved to be unsatisfactory.
British Pat. No. 1,385,743 describes an anti-skid control method for a vehicle having several brake-equipped vehicle wheels where releasing and reapplying brake pressure is controlled dependent on several parameters detected independently of each other. A control action is initiated, firstly, when either the speed signal of one wheel differs by a predetermined amount from the speed signal of the fastest rotating wheel, or, secondly, when the deceleration of a wheel exceeds a predetermined deceleration value, or thirdly, when a wheel speed signal decreases below a value corresponding to a computed reference speed. This prior art system controls the magnitude of air pressure supplied to the brakes dependent on the actual braking situation. In the second system, the detected decelerations of all wheels are compared to a predetermined fixed deceleration reference value and a trigger signal is generated when the deceleration signal of any of the wheels exceeds this reference value. This predetermined deceleration value is fixed and not self-adapting to road friction changes. Therefore this predetermined deceleration value may be chosen according to dry friction conditions and, hence, may be relatively high, which can result in an initiation of control action under wet or slippery surface conditions which is too late. But if instead of this the upper deceleration value chosen is relatively low which is representative of low friction conditions, the control actions will be initiated on dry surfaces at the instant at which the maximum obtainable deceleration has not yet been reached.
Another prior art anti-skid control method for a vehicle wheel is described in British Pat. No. 1,378,368. Therein, the deceleration of the wheel is detected and compared with a predetermined constant value for minimum deceleration and also with a constant reference value for maximum deceleration. A reservoir brake pressure is controlled such that, independent of the position of the brake operating means actuated by the driver, the reservoir brake pressure is released from the brake when the actual deceleration of that wheel exceeds the constant reference value for maximum deceleration and the reservoir pressure is fully reapplied to the brake if the actual deceleration of the wheel decreases below the predetermined constant value for minimum deceleration. The reservoir brake pressure is fully reapplied after the minimum deceleration value has been reached, in order to quickly increase the actual deceleration, to a value suitable for transferring relatively high frictional forces, such that the slip is in the range about 15 to 20%. After this time interval, if the desired deceleration has been reached, the brake pressure is slowly increased until the constant reference value for maximum deceleration is again exceeded. By this method, the frequency of operation of the system can be reduced to some extent and the available frictional force level can be somewhat better utilized. However, the use of a constant predetermined value for maximum deceleration implies the same disadvantages as identified above: the system is not self-adaptive to changes of road surface conditions, because always the same deceleration reference value initiates the system control actions.